In general, these filters are in the shape of a small frustoconical basket which is attached to the inside of the vessel in which it is implanted, downstream of the course which it is desired to filter; this is in general the vena cava arriving at the heart.
It is thus possible to stop, before they enter the heart, possible blood clots which can form and risk causing in particular embolisms.
One difficulty characteristic to this type of operation consists in positioning the filter correctly. In general, in order to introduce such a filter into the vessel, it is pushed therein by means of a tube which passes through the said vessel and whose diameter is less than that of the latter. When the filter reaches the end of the introduction tube, it is then released into the vessel and the expansion of its feet, which are often fitted with hooks, anchors it.
Such a "release" is in practice very tricky to control, and it has been shown in numerous cases that the basket filter in fact occupied, inside the vessel, a position other than the most favourable position with its axis substantially parallel to the axis of the vessel.
The invention which is the subject of Patent EP-A-188,927 provided a first solution to this centering problem, by providing for the feet of the filter to be provided, towards their free end, with appendages oriented substantially parallel to the cylindrical wall generated by a generator line parallel to the axis of the conical corolla of the filter.
Obviously, since the invention of this filter, research has continued.
During this research, it has in particular been shown that it could be advantageous to be able, in certain cases and if necessary, to make at least the actually filtering part of the filter (that is to say its part extending as far as the centre of the vessel) absorbable, while possibly keeping the most external part for attachment to the vessel unabsorbable, while still ensuring the best possible centering of the filter.
Known filters in the shape of a frustoconical basket have been shown to exhibit certain drawbacks linked in particular with the fact that if it was desired to make the central filtering part absorbable, the peripheral parts used for attachment risked becoming, after absorption of this central part, mutually disconnected, with, as a consequence, significant risks of detachment, with the obvious problems.